1. Field of Invention
The present invention relates to a surface acoustic wave device which further reduces the fluctuation of frequency to temperature change by using an in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis (hereinafter xe2x80x9cin-plane rotated ST cut quartz crystal platexe2x80x9d).
2. Description of Related Art
Related art surface acoustic wave devices exist in which IDT (Interdigital Transducer) electrodes are provided on the principal surface of a piezoelectric flat plate represented by a quartz crystal sheet, and multiple reflectors are provided at both ends of the IDT electrodes to oscillate a high frequency stably (hereinafter xe2x80x9cSAW resonatorxe2x80x9d).
An ST cut SAW resonator can be provided in which an ST cut quartz crystal plate is used as a piezo-electric flat plate to reduce the fluctuation of frequency to temperature change, and the X-axis direction of the ST cut quartz crystal plate is taken as the propagation direction of the acoustic wave.
FIG. 6 is a schematic sectional view showing the structure of an ST cut SAW resonator. As shown in FIG. 6, in the ST cut SAW resonator 1, an ST cut quartz crystal plate 2 is taken as the substrate, and IDT electrodes 3 are provided on the principal surface of the substrate. Comb teeth-like anodes 4 and cathodes 5 are alternately arranged in the IDT electrodes 3, and a surface acoustic wave is excited due to the piezoelectric effect of the quartz crystal plate by adding a high-frequency electric field between the anodes 4 and cathodes 5.
Multiple reflectors 6 are provided on both sides of the IDT electrodes 3 to reflect the surface acoustic wave, and the reflection of the surface acoustic wave emitted from the IDT electrodes 3 is performed by multiple short-circuit electrodes 7 formed in the reflectors 6. The anodes 4 and cathodes 5 in the IDT electrodes 3 and the short-circuit electrodes 7 in the reflectors 6 are arrayed in the X-axis direction of the ST cut quartz crystal plate 2, and the reflection of the surface acoustic wave in the short-circuit electrodes 7 is performed at positions of both edges of the electrodes.
In the ST cut SAW resonator 1 thus constructed, as shown in FIG. 6, the width and the pitch of the anodes 4 and cathodes 5 in the IDT electrodes 3 are defined as Lt, Pt, and the width and the pitch of the short-circuit electrodes 7 in the reflectors 6 are defined as Lr, Pr. The thickness of the anodes 4 and cathodes 5 is defined as Ht, and the thickness of the short-circuit electrodes 7 is defined as Hr.
FIG. 7 is a graph showing the reflection coefficient per short-circuit electrode of the ST cut SAW resonator. In the ST cut SAW resonator 1, if the reflection coefficient of a surface acoustic wave can be increased, it is possible to reduce the number of reflectors 6 and to miniaturize the resonator itself. FIG. 7 shows the value of Lt/Pt (hereinafter xe2x80x9cLr/Pr,xe2x80x9d is referred to as xe2x80x9cxcex7xe2x80x9d) on the horizontal axis, and shows the reflection coefficient per short-circuit electrode on the vertical axis, and shows how the reflection coefficient fluctuates with the value of the Ht/2Pt(≅Hr/2Pr).
When the reflection coefficient is considered, the Ht/2Pt and Hr/2Pr can be regarded as nearly the same value. 2Pt and 2Pr are nearly the same value, so 2Pt and 2Pr are defined as xcex. Therefore, in the present embodiment, Ht/2Pt and Hr/2Pr are not distinguished and are treated as the same value, i.e., H/xcex.
As shown in FIG. 7, for the ST cut SAW resonator 1, the reflection coefficient also increases with an increase in the xcex7 value, and the higher the H/xcex value, the greater the reflection coefficient for the relationship of H/xcex and reflection coefficient, as disclosed in Japanese Laid-Open Patent Application H2-260908.
In the ST cut SAW resonator 1, the thickness (H) of the anodes 4, cathodes 5 and short-circuit electrodes 7 is commonly set up so that the H/xcex values become about 0.03 from a viewpoint of obtaining objective temperature characteristics. The xcex7 value is set to 0.5, so that a relation of Pt=2Lt is established.
In the SAW resonator, a quartz crystal plate cut from the in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis is sometimes used to further reduce the frequency fluctuation caused by temperature change. However, the relationship between the xcex7, H/xcex value and the reflection coefficient have not yet been verified with the in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis.
The inventors studied the in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis, and discovered that it has characteristics quite different from a related art ST cut quartz crystal plate, and a regularity of the related art ST cut quartz crystal plate in which the reflection coefficient is increased by increasing the xcex7 and H/xcex values does not apply. Therefore, a problem arises that the reflection coefficient cannot be fully obtained, even if the regularity of the related art ST cut quartz crystal plate is applied to the in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis to increase the xcex7 and H/xcex values.
The present invention addresses the above problem, and provides a surface acoustic wave device which enables the reflection coefficient to be increased by grasping the characteristics of the in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis.
The present invention was discovered via various studies and the knowledge that the characteristics of the ST cut quartz crystal plate in a plane rotated around the Zxe2x80x2-axis differ from the characteristics of a related art ST cut quartz crystal plate.
Namely, the surface acoustic wave device relating to the present invention has one or more pairs of IDT electrodes to excite a Rayleigh wave arranged on a principal surface of an in-plane rotated ST cut quartz crystal plate and existing at a Euler angle (0, 113-135,xc2x1(40-49)), and the ratio Lt/Pt of width Lt of the IDT electrodes to pitch Pt of the IDT electrodes is less than 0.5. It is desirable that the Lt/Pt be 0.32xc2x10.1, and it is further desirable that the thickness of the IDT electrodes be taken as Ht and the Ht/2Pt be 0.06xc2x10.01.
Another surface acoustic wave device relating to the present invention has one or more pairs of IDT electrodes to excite a Rayleigh wave, and one or more reflectors to trap the Rayleigh wave arranged on the principal surface of the in-plane rotated ST cut quartz crystal plate existing at a Euler angle (0, 113-135,xc2x1(40-49)), and either or both of the ratio Lt/Pt of width Lt of the IDT electrodes to pitch Pt in the IDT electrodes and the ratio Lr/Pr of width Lr of the reflector to pitch Pr in the reflectors are less than 0.5.
It is desirable that either or both of the Lt/Pt and the Lr/Pr be 0.32xc2x10.1, and it is further desirable that the thickness of the IDT electrodes be taken as Ht and the thickness of the reflectors be taken as Hr, and either or both of Ht/2Pt and Hr/2Pr be 0.06xc2x10.01 in another surface acoustic wave device relating to the present invention.
Thus, an acoustic wave device which has an in-plane rotated ST cut quartz crystal plate around the Zxe2x80x2-axis is different from a surface acoustic wave device to which a related art ST cut quartz crystal plate is applied. If the Lt/Pt value in the IDT electrodes is decreased, the value of reflection coefficient is enhanced.
More specifically, it is desirable that the Lt/Pt value be less than the Lt/Pt value (0.5) generally applied to a related art surface acoustic wave device to which a related art ST cut quartz crystal plate is applied. Thus, setting the Lt/Pt value to be less than 0.5, enables the value of reflection coefficient to be enhanced and miniaturization of the device itself to be achieved.
Setting the Lr/Pr value of reflectors provided to be adjacent to the IDT electrodes to be less than 0.5 like Lt/Pt, enables the value of the reflection coefficient to be enhanced and promotes miniaturization of the device itself, in addition to the above effect. If both the value of Lt/Pt and the value of Lr/Pr are set to be less than 0.5, the value of reflection coefficient can be further enhanced, but the enhancement of the value of reflection coefficient can certainly be sought, even if either one of the value of Lt/Pt and value of Lr/Pr is set to be less than 0.5.
According to investigations of the inventors, setting either one or both of Lt/Pt and Lr/Pr to 0.32xc2x10.1, and further setting either one or both of Ht/2Pt and Hr/2Pr to 0.06xc2x10.01, enables the value of the reflection coefficient to be further enhanced, and miniaturization of device itself to be achieved.